Hypothalamic-Pituitary-Adrenal Axis Activity Cortisol and diurnal cortisol variation. Cortisol will be measured using self-administered saliva samples. Subjects will obtain samples on three consecutive days, at five intervals throughout each day (at waking, 45 minutes later, 5 pm, 9 pm and bedtime), in subjects’ own homes. A testing kit will be used by each subject consisting of 15 salivette tubes. Salivary cortisol has been found to be a reliable tool for investigations of hypothalamic-pituitary-adrenal axis activity.